Give each pane created in a tmux server a unique id (starting from 0),

put it in the TMUX_PANE environment variable and accept it as a
target. Suggested by and with testing and tweaks from Ben Boeckel.

cmd-list-panes.c

@@ -64,8 +64,8 @@ cmd_list_panes_exec(struct cmd *self, struct cmd_ctx *ctx)
 		}
 		size += gd->hsize * sizeof *gd->linedata;
 
-		ctx->print(ctx, "%u: [%ux%u] [history %u/%u, %llu bytes]%s%s",
-		    n, wp->sx, wp->sy, gd->hsize, gd->hlimit, size,
+		ctx->print(ctx, "%u: [%ux%u] [history %u/%u, %llu bytes] %%%u%s%s",
+		    n, wp->sx, wp->sy, gd->hsize, gd->hlimit, size, wp->id,
 		    wp == wp->window->active ? " (active)" : "",
 		    wp->fd == -1 ? " (dead)" : "");
 		n++;

cmd.c

@@ -118,6 +118,9 @@ struct client	*cmd_lookup_client(const char *);
 struct session	*cmd_lookup_session(const char *, int *);
 struct winlink	*cmd_lookup_window(struct session *, const char *, int *);
 int		 cmd_lookup_index(struct session *, const char *, int *);
+struct window_pane *cmd_lookup_paneid(const char *);
+struct session	*cmd_pane_session(struct cmd_ctx *,
+		    struct window_pane *, struct winlink **);
 struct winlink	*cmd_find_window_offset(const char *, struct session *, int *);
 int		 cmd_find_index_offset(const char *, struct session *, int *);
 struct window_pane *cmd_find_pane_offset(const char *, struct winlink *);
@@ -639,11 +642,64 @@ cmd_lookup_index(struct session *s, const char *name, int *ambiguous)
 	return (-1);
 }
 
+/*
+ * Lookup pane id. An initial % means a pane id. sp must already point to the
+ * current session.
+ */
+struct window_pane *
+cmd_lookup_paneid(const char *arg)
+{
+	const char	*errstr;
+	u_int		 paneid;
+
+	if (*arg != '%')
+		return (NULL);
+
+	paneid = strtonum(arg + 1, 0, UINT_MAX, &errstr);
+	if (errstr != NULL)
+		return (NULL);
+	return (window_pane_find_by_id(paneid));
+}
+
+/* Find session and winlink for pane. */
+struct session *
+cmd_pane_session(struct cmd_ctx *ctx, struct window_pane *wp,
+    struct winlink **wlp)
+{
+	struct session		*s;
+	struct sessionslist	 ss;
+	struct winlink		*wl;
+
+	/* If this pane is in the current session, return that winlink. */
+	s = cmd_current_session(ctx);
+	if (s != NULL) {
+		wl = winlink_find_by_window(&s->windows, wp->window);
+		if (wl != NULL) {
+			if (wlp != NULL)
+				*wlp = wl;
+			return (s);
+		}
+	}
+
+	/* Otherwise choose from all sessions with this pane. */
+	ARRAY_INIT(&ss);
+	RB_FOREACH(s, sessions, &sessions) {
+		if (winlink_find_by_window(&s->windows, wp->window) != NULL)
+			ARRAY_ADD(&ss, s);
+	}
+	s = cmd_choose_session_list(&ss);
+	ARRAY_FREE(&ss);
+	if (wlp != NULL)
+		*wlp = winlink_find_by_window(&s->windows, wp->window);
+	return (s);
+}
+
 /* Find the target session or report an error and return NULL. */
 struct session *
 cmd_find_session(struct cmd_ctx *ctx, const char *arg)
 {
 	struct session		*s;
+	struct window_pane	*wp;
 	struct client		*c;
 	char			*tmparg;
 	size_t			 arglen;
@@ -654,6 +710,10 @@ cmd_find_session(struct cmd_ctx *ctx, const char *arg)
 		return (cmd_current_session(ctx));
 	tmparg = xstrdup(arg);
 
+	/* Lookup as pane id. */
+	if ((wp = cmd_lookup_paneid(arg)) != NULL)
+		return (cmd_pane_session(ctx, wp, NULL));
+
 	/* Trim a single trailing colon if any. */
 	arglen = strlen(tmparg);
 	if (arglen != 0 && tmparg[arglen - 1] == ':')
@@ -684,6 +744,7 @@ cmd_find_window(struct cmd_ctx *ctx, const char *arg, struct session **sp)
 {
 	struct session		*s;
 	struct winlink		*wl;
+	struct window_pane	*wp;
 	const char		*winptr;
 	char			*sessptr = NULL;
 	int			 ambiguous = 0;
@@ -704,6 +765,14 @@ cmd_find_window(struct cmd_ctx *ctx, const char *arg, struct session **sp)
 		return (s->curw);
 	}
 
+	/* Lookup as pane id. */
+	if ((wp = cmd_lookup_paneid(arg)) != NULL) {
+		s = cmd_pane_session(ctx, wp, &wl);
+		if (sp != NULL)
+			*sp = s;
+		return (wl);
+	}
+
 	/* Time to look at the argument. If it is empty, that is an error. */
 	if (*arg == '\0')
 		goto not_found;
@@ -998,6 +1067,14 @@ cmd_find_pane(struct cmd_ctx *ctx,
 		return (s->curw);
 	}
 
+	/* Lookup as pane id. */
+	if ((*wpp = cmd_lookup_paneid(arg)) != NULL) {
+		s = cmd_pane_session(ctx, *wpp, &wl);
+		if (sp != NULL)
+			*sp = s;
+		return (wl);
+	}
+
 	/* Look for a separating period. */
 	if ((period = strrchr(arg, '.')) == NULL)
 		goto no_period;

server.c

@@ -143,6 +143,7 @@ server_start(void)
 	log_debug("server started, pid %ld", (long) getpid());
 
 	ARRAY_INIT(&windows);
+	RB_INIT(&all_window_panes);
 	ARRAY_INIT(&clients);
 	ARRAY_INIT(&dead_clients);
 	RB_INIT(&sessions);

tmux.1

@@ -449,6 +449,17 @@ select-window -t:+2
 When dealing with a session that doesn't contain sequential window indexes,
 they will be correctly skipped.
 .Pp
+.Nm
+also gives each pane created in a server an identifier consisting of a
+.Ql %
+and a number, starting from zero.
+A pane's identifier is unique for the life of the
+.Nm
+server and is passed to the child process of the pane in the
+.Ev TMUX_PANE
+environment variable.
+It may be used alone to target a pane or the window containing it.
+.Pp
 .Ar shell-command
 arguments are
 .Xr sh 1

tmux.h

@@ -779,6 +779,8 @@ struct window_mode {
 
 /* Child window structure. */
 struct window_pane {
+	u_int		 id;
+
 	struct window	*window;
 	struct layout_cell *layout_cell;
 
@@ -821,8 +823,10 @@ struct window_pane {
 	void		*modedata;
 
 	TAILQ_ENTRY(window_pane) entry;
+	RB_ENTRY(window_pane) tree_entry;
 };
 TAILQ_HEAD(window_panes, window_pane);
+RB_HEAD(window_pane_tree, window_pane);
 
 /* Window structure. */
 struct window {
@@ -1825,8 +1829,11 @@ int	 screen_check_selection(struct screen *, u_int, u_int);
 
 /* window.c */
 extern struct windows windows;
+extern struct window_pane_tree all_window_panes;
 int		 winlink_cmp(struct winlink *, struct winlink *);
 RB_PROTOTYPE(winlinks, winlink, entry, winlink_cmp);
+int		 window_pane_cmp(struct window_pane *, struct window_pane *);
+RB_PROTOTYPE(window_pane_tree, window_pane, tree_entry, window_pane_cmp);
 struct winlink	*winlink_find_by_index(struct winlinks *, int);
 struct winlink	*winlink_find_by_window(struct winlinks *, struct window *);
 int		 winlink_next_index(struct winlinks *, int);
@@ -1861,6 +1868,7 @@ struct window_pane *window_pane_previous_by_number(struct window *,
 u_int		 window_pane_index(struct window *, struct window_pane *);
 u_int		 window_count_panes(struct window *);
 void		 window_destroy_panes(struct window *);
+struct window_pane *window_pane_find_by_id(u_int);
 struct window_pane *window_pane_create(struct window *, u_int, u_int, u_int);
 void		 window_pane_destroy(struct window_pane *);
 int		 window_pane_spawn(struct window_pane *, const char *,

window.c

@@ -56,6 +56,10 @@
 /* Global window list. */
 struct windows windows;
 
+/* Global panes tree. */
+struct window_pane_tree all_window_panes;
+u_int	next_window_pane;
+
 void	window_pane_read_callback(struct bufferevent *, void *);
 void	window_pane_error_callback(struct bufferevent *, short, void *);
 
@@ -67,6 +71,14 @@ winlink_cmp(struct winlink *wl1, struct winlink *wl2)
 	return (wl1->idx - wl2->idx);
 }
 
+RB_GENERATE(window_pane_tree, window_pane, tree_entry, window_pane_cmp);
+
+int
+window_pane_cmp(struct window_pane *wp1, struct window_pane *wp2)
+{
+	return (wp1->id - wp2->id);
+}
+
 struct winlink *
 winlink_find_by_window(struct winlinks *wwl, struct window *w)
 {
@@ -495,6 +507,16 @@ window_printable_flags(struct session *s, struct winlink *wl)
 	return (xstrdup(flags));
 }
 
+/* Find pane in global tree by id. */
+struct window_pane *
+window_pane_find_by_id(u_int id)
+{
+	struct window_pane	wp;
+
+	wp.id = id;
+	return (RB_FIND(window_pane_tree, &all_window_panes, &wp));
+}
+
 struct window_pane *
 window_pane_create(struct window *w, u_int sx, u_int sy, u_int hlimit)
 {
@@ -503,6 +525,9 @@ window_pane_create(struct window *w, u_int sx, u_int sy, u_int hlimit)
 	wp = xcalloc(1, sizeof *wp);
 	wp->window = w;
 
+	wp->id = next_window_pane++;
+	RB_INSERT(window_pane_tree, &all_window_panes, wp);
+
 	wp->cmd = NULL;
 	wp->shell = NULL;
 	wp->cwd = NULL;
@@ -555,6 +580,8 @@ window_pane_destroy(struct window_pane *wp)
 		bufferevent_free(wp->pipe_event);
 	}
 
+	RB_REMOVE(window_pane_tree, &all_window_panes, wp);
+
 	if (wp->cwd != NULL)
 		xfree(wp->cwd);
 	if (wp->shell != NULL)
@@ -569,7 +596,7 @@ window_pane_spawn(struct window_pane *wp, const char *cmd, const char *shell,
     const char *cwd, struct environ *env, struct termios *tio, char **cause)
 {
 	struct winsize	 ws;
-	char		*argv0;
+	char		*argv0, paneid[16];
 	const char	*ptr;
 	struct termios	 tio2;
 
@@ -616,6 +643,8 @@ window_pane_spawn(struct window_pane *wp, const char *cmd, const char *shell,
 
 		closefrom(STDERR_FILENO + 1);
 
+		xsnprintf(paneid, sizeof paneid, "%%%u", wp->id);
+		environ_set(env, "TMUX_PANE", paneid);
 		environ_push(env);
 
 		clear_signals(1);